Current source circuit

ABSTRACT

A small value current source circuit utilized in bipolar integrated circuits. A reference potential is level-shifted by a first level shift circuit and applied to one input of a differential amplifier. The reference potential is also level-shifted by a second level shift circuit and applied to the other input of the differential amplifier. A constant current source circuit supplies currents both proportional to the currents flowing in the differential amplifier and of mutually different values to the first and second level shift circuits. The differential amplifier amplifies the difference of the input voltages and produces an output current.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a current source circuit, inparticular, to a minute current source circuit used by a bipolar IC(integrated circuit).

2. Description of Prior Art

A conventional and widely used minute current source is shown in FIG. 1.This is basically a current mirror and, as commonly known, therelationship between input and output is as follows.

    I.sub.in =I.sub.out ·exp (I.sub.out R.sub.1 /V.sub.t) (1)

(V_(t) =kT/q)

V_(t) : Thermal voltage

k: Boltzmann's constant

T: Coupling temperature (°K.)

q: Electron quantum charge

For example, in order to obtain a 0.1 μA output current from a 10 μAinput current, when coupling temperature T is a room temperature ofappoximately 300° K., since the thermal voltage Vt is 26 mV, accordingto Formula 1, the value of resistance R1 becomes 1.2 MΩ. If this type ofcurrent mirror circuit is formed as part of an integrated circuit, asthe resistance value increases, the space occupied by the resistance inthe integrated circuit increases, while the resistance value accuracydeclines. When these factors are considered, a resistance value of 1.2MΩ is excessively high for forming part of an integrated circuit.

As can also be understood from Formula 1, the input/output relationshipis not linear, and even when the temperature coefficient of resistanceR₁ is taken as 0, a temperature factor exists. This condition isindicated in FIG. 2.

The circuit indicated in FIG. 3 is also well used. In this case, theinput/output relationship is as follows.

    I.sub.in =I.sub.out ·exp (-I.sub.out R.sub.2 /V.sub.t) (2)

Under the same temperature conditions as Formula 1, in order to obtain a0.1 μA output current from a 10 μA input current, according to Formula2, resistance R₂ is 12 KΩ. Although a high resistance, such as R₁ inFIG. 1, is not needed, as indicated in FIG. 4, the input/outputrelationship does not increase simply, but in the range of actual use asa minute current source, the relation is such that when the inputcurrent increases, the output current decreases. Also the resistance R₂voltage drop increases, and when transistor Q1 enters saturation, theconditions of Formula 2 are no longer met and even when the temperaturecoefficient of resistance R₂ is taken as 0, a temperature factor exists.

As described above, in attempting to achieve a minute current sourcewith the conventional circuit of FIG. 1, a high resistance is neededthat is too large to incorporate into an IC, while the expanded chipsize raises the cost. Resistance accuracy is also deficient. In theconventional circuit of FIG. 3, which seeks to avoid these problems, asthe input current increases, the output current decreases. With respectto small input current variation, the output current variation is large,and there is risk of transistor saturation.

In addition, a common point of both the circuits of FIGS. 1 and 3 isthat the input/output relationship is not linear, and an output currentproportional to the input current cannot be obtained. As they alsopossess a temperature response, they have the disadvantage in thatchanges in temperature result in changes in output current.

SUMMARY OF THE INVENTION

An objective of the present invention is to provide a minute currentsource circuit that does not use the above mentioned resistance,possesses a linear input/output current relationship, and wherein thisrelationship does not have a temperature response.

A current source circuit in accordance with this invention comprises:

a differential amplifier that includes at least a first and a secondtransistor, for amplifying a difference in voltage applied to eachtransistor base, the voltage difference being output from a collector ofsaid first transistor as an output current;

a first level shift circuit, including at least one first PN junctionconnected between a reference potential and said first transistor base,for level-shifting the reference potential to a first voltage dropproduced across said first PN junction to apply the first voltage dropto said first transistor base;

a second level shift circuit, including second PN junction the number ofwhich is equivalent to that of the first PN junction, connected betweensaid reference potential and said second transistor base, forlevel-shifting the potential difference to a second voltage dropproduced across said second PN junction to apply the second voltage dropto said second transistor base;

a first constant current circuit for supplying a first currentproportional to the current flowing in said differential amplifier tosaid first level shift circuit; and

a second constant current circuit for supplying a second currentproportional to the current flowing in said differential amplifier anddifferent from said first current to said second level shift circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a conventional current source circuit,

FIG. 2 shows the current response of the conventional current sourcecircuit shown in FIG. 1,

FIG. 3 shows another conventional current source circuit,

FIG. 4 shows the current response of the conventional current sourcecircuit shown in FIG. 3,

FIG. 5 shows a circuit in accordance with a first embodiment of thisinvention,

FIG. 6 shows the current response of the first embodiment of thisinvention,

FIG. 7 shows a circuit in accordance with a second embodiment of thisinvention,

FIG. 8 shows a circuit in accordance with a third embodiment of thisinvention,

FIG. 9 shows a circuit in accordance with a fourth embodiment of thisinvention,

FIG. 10 shows a circuit in accordance with a fifth embodiment of thisinvention, and

FIG. 11 shows a circuit in accordance with a sixth embodiment of thisinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 5 indicates a first embodiment of this invention as a currentsource circuit. In FIG. 5, the base of a transistor Q₁ is connected to areference voltage source V_(bias), and the emitter of the transistor Q₁is connected to the base of a transistor Q₃. Likewise, the emitter oftransistor Q₃ is connected to the base of transistor Q₅. Thisconfiguration continues to a transistor Q_(2M+1), with quantity M (M isan integral number of 1 or more) transistors consist of a Darlingtoncircuit.

The respective emitters of the transistors Q₁, Q₃ . . . Q_(2M-1) areconnected to current sources I₁, I₃, . . . , I_(2M-1). These transistorsand current sources consist of a first level shift circuit 1.

Likewise, the respective emitters of the transistors Q₂, Q₄ . . . ,Q_(2M) are connected to current sources I₂, I₄, . . . , I_(2M). Thesetransistors and current sources consist of a second level shift circuit2.

The base of a transistor Q_(2M+1) is connected to the emitter of thetransistor Q_(2M-1), while the base of a transistor Q_(2M+2) isconnected to the emitter of the transistor Q_(2M). The emitter of thetransistor Q_(2M+1) is connected via quantity L (L is an integral numbergreater than 0) of diodes Q_(2M+3) -Q_(2M+2L+1) to a current sourceI_(in), while the emitter of the transistor Q_(2M) is connected viaquantity L of diodes Q_(2M+4) -Q_(2M+2L+2) to the current source I_(in).These transistors, diodes and current source consist of a differentialamplifier 3.

The outputs of these current sources I₁, I₂, . . . , I_(2M) areproportional to the output of the current source I_(in). Theproportional constants of the current sources I₁, I₂, . . . , I_(2M)with respect to the current source I_(in) are C₁, C₂, . . . , C_(2M)respectively.

The emitter area ratios of the transistors Q₁, Q₂, Q₃, Q₄, . . . ,Q_(M+1), Q_(M+2) and diodes Q_(2M+3), Q_(2M+4), . . . , Q_(2M+2L+1),Q_(2M+2L+2) are respectively N₁, N₂, N₃, N₄, . . . , N_(2M+1), N_(2M+2)and N_(2M+3), N_(2M+4), . . . , N_(2M+2L+1), N_(2M+2L+2).

In accordance with the above mentioned circuit configuration, an outputvoltage of the reference voltage source V_(bias) is level-shifted by thefirst level shift circuit 1 and applied to the base of the bipolartransistor Q_(2M+1) of the differential amplifier circuit 3. The outputvoltage of the reference voltage V_(bias) is also level-shifted by thesecond level shift circuit 2 and applied to the base of the bipolartransistor Q_(2M+2) of the differential amplifier circuit 3.

A difference in current density arises according to each transistoremitter area ratio in the currents flowing through the first and secondlevel shift circuits 1 and 2. This results in a difference in voltageapplied to the bases of the two bipolar transistors Q_(2M+1) andQ_(2M+2) of the differential amplifier circuit 3, by which the collectorcurrents of these bipolar transistors are controlled. A collectorcurrent I_(out) of the transistor Q_(2M+1) thus controlled then appearsat an output terminal 4.

Following is a description of the operation of the circuit shown in FIG.5.

As commonly known, the voltage V_(be) between the base and emitter of atransistor can be expressed as V_(be) =V_(t) I_(n) (I_(c) /N_(I) s),wherein V_(t) is the thermal voltage, Ic is the collector current, N isthe emitter area ratio, and I_(s) is the collector saturation current.

In FIG. 5, the following voltage formula can be composed for thebase-to-emitter closed circuit comprising the transistors Q₁, Q₃, . . ., Q_(2M+1), diodes Q_(2M+3), Q_(2M+5), . . . , Q_(2M+2L+1), Q_(2M+2L+2),. . . , Q_(2M+6), Q_(2M+4), and transistors Q_(2M+2), Q_(2M), . . . ,Q₄, Q₂. ##EQU1##

A collector current I_(c) flowing in each transistor of the first andsecond level shift circuit 1 and 2 is, because the proportionalconstants of the current sources connected to these transistors and withrespect to the current source I_(n) are respectively C₁, . . . , C_(2M),expressed as follows.

    I.sub.c (Q.sub.1)=C.sub.1 I.sub.in, I.sub.c (Q.sub.2)=C.sub.2 I.sub.in, I.sub.c (Q.sub.3)=C.sub.3 I.sub.in,

    I.sub.c (Q.sub.4)=C.sub.4 I.sub.in, . . . , I.sub.c (Q.sub.2M-1)=C.sub.2M-1 I.sub.in,

    I.sub.c (Q.sub.2M)=C.sub.2M I.sub.in                       (4)

Furthermore, the following relationships exist.

    I.sub.c (Q.sub.2M+1)=I.sub.c (Q.sub.2M+3)=

    . . .=I.sub.c (Q.sub.2M+2L+1)=I.sub.out . . .              (5)

    I.sub.c (Q.sub.2M+2)=I.sub.c (Q.sub.2M+4)=

    . . .=I.sub.c (Q.sub.2M+2L+2)=I.sub.in -I.sub.out          (6)

Therefore, I_(out) can be derived from Formulas (3)-(6) as follows.##EQU2## In the above,

    N=(N.sub.2 ·N.sub.4 ·N.sub.6 . . . N.sub.2M+2L+2)/(N.sub.1 N.sub.3 N.sub.5 . . . N.sub.2M+2L+1) (8)

    C=(C.sub.1 -C.sub.3 -C.sub.5 . . . C.sub.2M-1)/(C.sub.2 -C.sub.4 -C.sub.6 . . . C.sub.2M)                                             (9)

In a minute current source, although it is necessary to set the ##EQU3##of Formula (7) to a value sufficiently smaller than 1, from Formulas (8)and (9), it is fully possible to set N and C to values larger than 1,for example, to 100 or 1000. Also, since in an actual circuit, even alarge value for L is less than 10, ##EQU4## the value is sufficientlyless than 1.

As can be understood from Formula (7), the relationship between theinput current I_(in) and the output current I_(out) is linear, and iscompletely independent of resistance and temperature. The input/outputresponse of this circuit is as shown in FIG. 6.

Next is a description of a second embodiment of this invention as acurrent source circuit with reference to FIG. 7.

With respect to FIG. 5, in the current source circuit of FIG. 7, L=0,M=1 and current sources comprise a current mirror circuit.

In this circuit, when the emitter area ratios of transistors Q₁, . . . ,Q₄, Q₅₀, . . . , Q₈₀ are taken in sequence as N₁ -N₈ and N₆ =N₈, sinceI_(c) (Q₆₀)=I_(in), M=1 and L=0 can be substituted in Formulas (7) and(8) to yield the following relations.

    I.sub.out ={1/(1+NC)}I.sub.in                              (7-1)

    N=(N.sub.2 N.sub.4)/(N.sub.1 N.sub.3)                      (8-1)

    C=C.sub.1 /C.sub.2                                         (9-1)

As N₆ =N₈ =1, and the relations of C₁ and C₂ are C₁ =N₅ and C₂ =N₇, thefollowing is derived from Formula 9-1.

    C=N.sub.5 /N.sub.7                                         (9-1')

Consolidating these formulas yields the following.

    I.sub.out =[1/{1+(N.sub.2 N.sub.4 N.sub.6)/(N.sub.1 N.sub.3 N.sub.7 }]I.sub.in                                                (10)

In the above, N₆ =N₈ =1. For example, when N₁ =N₃ =N₆ =N₇ =N₈ =1 and N₂=N₄ =N₅ =10, I_(out) =(1/1001) I_(in). The output current I_(out)becomes 1/1001 the magnitude of the input current I_(in). This outputcurrent is directly proportional to the input current and is independentof resistance or temperature factors.

Following is a description of a third embodiment of this invention as acurrent source circuit with reference to FIG. 8.

Although M=1 and L=0 in the same manner as the FIG. 7 circuit, in theFIG. 8 circuit, transistors Q₁ and Q₂ are used as diodes. The basicoperation is the same as the FIG. 5 circuit and in this case as well,with the Q₁ -Q₈₀ emitter area ratio at N₁ -N₈ as N₆ =N₈ =1, theconditions of Formula 10 are composed in the same manner as the FIG. 7circuit.

Although in this case, both Q₁ and Q₂ are used as diodes, it is alsopossible to use only one of these as a diode. For example, if only Q₁ isa diode and Q₂ is a transistor, the emitter of the transistor Q₂ isconnected to the base of the transistor Q₄, and the base of thetransistor Q₂ is connected to the anode of the diode Q₁, and thecollector of the transistor Q₄ is connected to the power source V_(cc).This configuration as well forms the conditions applicable to Formula10.

Next is a description of a fourth embodiment of this invention as acurrent source circuit with reference to FIG. 9.

In the FIG. 9 circuit, the polarities of the diodes Q₁ and Q₂ arereversed with respect to the FIG. 8 circuit and a current mirror circuitis provided in place of the reference voltage source V_(bias). Theemitter area ratio of diodes Q₁ and Q₂ and transistors Q₃, . . . , Q₁₀₀is taken in sequence as N₁ -N₁₀.

In this case, the following voltage formula can be composed for thebase-to-emitter closed circuit of diode Q₁, transistors Q₃ and Q₄, anddiode Q₂.

    V.sub.t I.sub.n (I.sub.c (Q.sub.1)/N.sub.1 I.sub.s)-V.sub.t I.sub.n (I.sub.c (Q.sub.3)/N.sub.3 I.sub.s)+

    V.sub.t I.sub.n (I.sub.c (Q.sub.4)/N.sub.4 I.sub.s)-V.sub.t I.sub.n (I.sub.c (Q.sub.2)/N.sub.2 I.sub.s)=0                     (11)

The following formula also applies.

    I.sub.c (Q.sub.1)=(N.sub.5 N.sub.10 /N.sub.8 N.sub.9)I.sub.in, I.sub.c (Q.sub.2)=(N.sub.7 N.sub.10 /N.sub.8 N.sub.9)I.sub.in,

    I.sub.c (Q.sub.3)=I.sub.out, I.sub.c (Q.sub.4)=I.sub.c (Q.sub.6)-I.sub.out,

    I.sub.c (Q.sub.6)=(N.sub.6 /N.sub.8)I.sub.in               (12)

When N₆ =N₈ =1, I_(out) can be derived from Formulas 11 and 12 asfollows.

    I.sub.out =[1/{1+(N.sub.1 N.sub.4 N.sub.7 /N.sub.2 N.sub.3 N.sub.5)}]I.sub.in                                        (13)

As can be understood from comparing Formulas (13) and (10), althoughthey differ in form, they are the same and yield the same results.

A fifth embodiment of this invention is shown in FIG. 10.

FIG. 10 is a case where M=2 and L=0, and a diode Q₁₀₁ is provided inplace of the reference voltage source V_(bias).

The diode Q₁₀₁ and transistor Q₁, . . . , Q₁₀₀ area ratio is N₁ -N₁₀ andwhen N₈ =N₁₀ =1, since I_(c) (Q₈₀)=I_(in), M=2 and L=0 can besubstituted in Formulas (7) and (8) to yield the following.

    I.sub.out ={1/(1+N.sub.k)}I.sub.in                         (7-2)

    N=(N.sub.2 N.sub.4 N.sub.6)/(N.sub.1 N.sub.3 N.sub.5)      (8-2)

    C=(C.sub.1 C.sub.3)/(C.sub.2 C.sub.4)                      (9-2)

Since N₈ =N₁₀ =1, the relationship to C₁ -C₆ is C₁ =C₃ =N₇, and C₂ =C₄=N₂, the following is obtained from Formula 9-2.

    C=(N.sub.7 /N.sub.2).sup.2                                 (9-2')

These formulas can be consolidated as follows.

    I.sub.out =[1/{1+(N.sub.2 N.sub.4 N.sub.6 N.sub.7.sup.2)/N.sub.1 N.sub.3 N.sub.5 N.sub.9.sup.2)}]I.sub.in                          (14)

In the above, N₈ =N₁₀ =1.

For example, when N₁ =N₃ =N₅ =N₈ =N₉ =N₁₀ =1, and N₂ =N₄ =N₆ =N₇ =4,I_(out) =(1/1025) I_(in). The output current I_(out) becomes 1/1025 themagnitude of the input current I_(in), independently of resistance andtemperature factors.

A sixth embodiment of this invention as a current source circuit isshown in FIG. 11. In the FIG. 11 example, M=3 and L=1, and the emitterarea ratios of the diodes and transistors Q₁ -Q₁₄ are N₁ -N₁₄. When N₁₃=N₁₅ =1, since I_(c) (Q₁₃)=I_(in), M=3 and L=1 can be substituted inFormulas (7) and (8) to yield the following.

    I.sub.out ={1/(1+NC)}I.sub.in                              (7-3)

    N=(N.sub.2 N.sub.4 N.sub.6 N.sub.8 N.sub.10)/(N.sub.1 N.sub.3 N.sub.5 N.sub.7 N.sub.9)                                          (8-3)

    C=(C.sub.1 ·C.sub.3 ·C.sub.5)/(C.sub.2 ·C.sub.4 ·C.sub.6)                                        (9-3)

Since N₁₃ =N₁₅ =1, the relationship of C₁ -C₆ is C₁ =C₃ =N₁₁, C₅ =N₁₂and C₂ =C₄ =C₆ =N₁₄. The following is obtained from Formula (7-3).

    k=N.sub.11.sup.2 N.sub.12 /N.sub.14.sup.3                  (9-3')

By consolidating these formulas, the following relationship is obtained.

    I.sub.out =[1/{1+(N.sub.2 N.sub.4 N.sub.6 N.sub.8 N.sub.10 N.sub.11.sup.2 N.sub.12)/(N.sub.1 N.sub.3 N.sub.5 N.sub.7 N.sub.9 N.sub.14.sup.3)}]I.sub.in                                 (15)

In the above, N₁₃ =N₁₅ =1.

The relationship between I_(out) and I_(in) is determined only by theemitter area ratio, independently of resistance and temperature.

As described in the foregoing, as a result of this invention, a currentsource circuit can be realized wherein the relationship between an inputcurrent and an output current is determined solely by the transistorarea ratio and independently of the input/output current, resistance andtemperature. Furthermore, since the input/output current relationship islinear, an output current proportional to the input current can beobtained. In addition, this advantage is realized even if the inputcurrent comprises a bias current and current variation component, thusenabling applications as a superbly linear current attenuator.

What is claimed is:
 1. A current source circuit comprising:adifferential amplifier including at least a first and a secondtransistor for amplifying a difference in voltage applied to eachtransistor base, the voltage difference being output from a collector ofsaid first transistor as an output current; a first voltage controlcircuit including at least one first PN junction connected between areference potential and said first transistor base, said referencepotential being applied to said first PN junction to cause a firstvoltage drop across said PN junction, the first voltage drop beingapplied to said first transistor base; a second voltage control circuitincluding at least one second PN junction connected between saidreference potential and said second transistor base, the referencepotential being applied to said second PN junction to cause a secondvoltage drop across said second PN junction, the second voltage dropbeing applied to said second transistor base, wherein a first number ofPN junctions including a PN junction of said first transistor and the PNjunction included in said first voltage control circuit is equivalent toa second number of PN junctions including a PN junction of said secondtransistor and the PN junction included in said second voltage controlcircuit; a first constant current circuit for supplying a first currentproportional to the output current flowing in said differentialamplifier, to said first voltage control circuit; and a second constantcurrent circuit for supplying a second current proportional to theoutput current flowing in said differential amplifier and different fromsaid first current to said second voltage control circuit.
 2. A currentsource circuit in accordance with claim 1 whereinsaid first voltagecontrol circuit comprises a plurality of PN junction stages.
 3. Acurrent source circuit in accordance with claim 1 whereina currentdensity corresponding to said first voltage drop flowing in the PNjunction of said first voltage control circuit is greater than a currentdensity corresponding to said second voltage drop flowing in the PNjunction of said second voltage control circuit.
 4. A current sourcecircuit in accordance with claim 1 whereinan emitter area of said firsttransistor is different from an emitter area of said second transistor.5. A current source circuit in accordance with claim 1 whereinsaid firstand second constant current circuits include current mirror circuits.